The present invention relates to niobium tin tape magnet coils which has been epoxy impregnated and do not require helium cooling for stability.
Niobium tin tape superconductors have been made by several processes, namely the GE/IGC tin dip-reaction process by Benz, CVD process by RCA, or the plasma spray process by Union Carbide. These tapes have been used extensively to make high field magnets which are cooled by pool boiling in liquid helium or forced convection of gaseous helium to stabilize the superconductor against flux jumps. Flux jumps can be understood by considering what happens when a magnetic field occurs perpendicular to a face of a superconducting tape. The magnetic field induces currents in the tape according to Lenz's Law, which try to screen the superconducting tape from the field. As long as the induced currents are below the critical current of the material, the currents persist. If the field increases or a section of the superconducting tape is externally heated, and the critical current is exceeded, heat is generated by the flowing current and the current decay. The flux then penetrates further into the superconducting tape inducing additional currents in the tape. Since critical current density of a superconductor generally decreases with increasing temperature, a temperature rise can lead to further flux penetration, which generates heat, leading to a still greater temperature rise. This thermal magnetic feedback can under some conditions lead to a thermal runaway, a catastrophic flux jump. Not all flux jumps lead to thermal runaway. If a flux jump occurs and the current induced does not exceed the critical current density, the flux jump stops. Direct cooling of the superconducting tape with helium has been widely accepted as the only feasible method to stabilize tape against flux jumps. Because of the inherent flux jump instability of niobium tin tapes and the complicated method of cooling tape magnets which requires a porous structure and the use of helium, the use of superconductive tape magnets has been rather limited and never commercialized in spite of the fact that niobium tin tape is the lowest cost superconductor. Instead, the effort was concentrated in making multifilamentary niobium tin superconductor wire, which due to the fine subdivision of the superconductor is inherently stable, but many times more expensive.
It is an object of the present invention to provide a coil of superconductive tape that does not require helium cooling for stability.
It is a further object of the present invention to provide free standing coil of superconductive tapes suitable for use in magnetic resonance imaging magnets cooled by refrigeration.